Separation of hydrocarbons



J. B BAIRD ETAL SEPARATIONv OF' HYDROCARBONS Feb. 2, 1954 Filed 0G12. 27, 1949 mnt Jo/m lBG/ra' Ne/Jon C. Turner f/v VEA/rom BYMW A TTORNE YS Patented Feb. 2,

SEPARATION OF HYDROCARBONS John B. Baird and Nelson C. Turner, Houston,

Tex., assignors to Hudson Engineering Corporation, Houston, Tex., a corporation of Texas Application October 27, 1949, Serial No. 123,882

3 Claims.

l This invention relates to improvements in absorption separation methods of recovering selected heavy constituents from a mixture of hydrocarbon paraiiins and refers more particularly to the fractionation of the overhead product from the rectification still of such installations to substantially denude this product of all but acceptable traces of the less valuable lighted hydrocarbon material.

The normally available mixtures of paraffin hydrocarbons, as for example natural gas, usually contain many constituents ranging in molecular weight from methane to nonane and even heavier. The heavier ones of these hydrocarbons are more valuable than the lighter constituents and therefore, it is the present day practice to separate the lighter hydrocarbon materials from the heavier ones. In some installations the ethane and heavier hydrocarbons are desired, and in other installations propane and higher molecular weight hydrocarbons are desired. Usually traces vof the light hydrocarbons in the order of about 5% of the lightest of the heavier hydrocarbons are commercially acceptable. The terms paraffin or any particular paraiiin when used herein are intended to include the straight chain and isomers, and also the invention is equally applicable to their corresponding olens.

It has heretofore been the practice to substantially separate the desirable heavier hydrocarbon constituents of the gases from the lighter material by absorption methods and then to remove all but the last traces of the lighter constituents or constituent by fractionation. In

such fractionation processes, it has been the practice to cool and condense at least a portion of the overhead product from the fractionator and return it to the upper portion of the fractionator column as a liquid reflux. The apparatus required to effect this circulation of reflux liquid has a high initial cost and is expensive to operate.

In most of the high pressure absorption installations in present day operation, a liquid product is removed from the feed stock by retrogradeV condensation that is very rich in the heavier parains but which does contain small but 0bjectionable amounts of the lighter hydrocarbons. It has been proposed to contact this liquid with the product recovered from the rich oil to further reduce the light paraflin content of both these materials and this has produced desirable results but has not been effective to denude the liquid product of thevundesirable light-material toi the l extent that v further demethanizationor deethanization is not required.

An object of this invention is to provide an economical method of separating the desirable heavy hydrocarbon parafns from a mixture of hydrocarbon parafins containing both light and heavy constituents to produce a product that is sufficiently free of the light material that the product need not be further deethanized or demethanizedas the case may be.

Another object is to provide an economical method of fractionating a mixture of hydrocarbon parairlns of predominately selected heavy constituents to sufficiently free it o f the light parafiins that the product need not be further deethanized or demethanized as the case may be.

A further object is to provide in a method of separating selected heavy hydrocarbons from a mixture of hydrocarbon paraflins, of the absorption class, a method of fractionating the overhead product from the still for rectifying the rich oil wherein the condensatel from the gas feed stream is employed as a reflux liquid to substantially denude both the reflux liquid and the still product of the lighter hydrocarbons.

Still another object is to provide in a method of separating selected heavy hydrocarbons from a mixture of hydrocarbon paraii'ins, ofthe absorption class, a method of fractionating the overhead product from the still for rectifying the rich oil wherein the retrograde condensation product from the gas feed stream is employed as a reflux liquid to substantially denude both the reflux liquid and the still product of the lighter hydrocarbon material, and wherein the overhead product from the fractionator is recycled to the absorption process where any escaping desirable heavy hydrocarbons may be stripped therefrom.

A still further object is to provide a method of substantially denuding the overhead still product from an absorption separation installation of the less valuable light hydrocarbon material in a fractionator column wherein a condensate obtained from the feed gas stream is employed asa reflux liquid and is itself denuded of its content of light constituents.

Other and further objects of this invention will appear from the following description.

In the accompanying drawings which constitute a part of the instant specification and are to be read in conjunction therewith, the single iigure is a schematic flow 4diagram illustrating an absorption separation installation constituting an embodiment of this invention. v

This invention, in general, is concerned with the portion of an absorption separation process connection with any conventional absorption separation method and installation wherein, for

instance, a plurality of absorption vessels such" as an absorber and reabsorber maybe employed and wherein the condensate separated from the feed gas stream may be enriched byhashing into several successive reduced pressure stages before it is employed as the reiiux" liquid in'accor'dance with the improvement of this invention. All of'vv such methods and equipment are Well known and understood by those skilled inthe art. l A

Referring `to ther drawings, the numeral i designatesan absorber vessel which may be of the usual-bubble tray type, the rectifier still for the richv oil from the absorber is shown at 2, and the fraetionator tower from which the nal product free from the lighter material is obtained is shown at f3; The feed stream for the process may be obtained from any ofthe usual sources andisintroduced tothe installation through conduit;4; Vessel-5 provides an accumulator for the condensation'product which is obtained from the feedstream either by the reduction of pressure in theicase of a high pressure gas or the increase in pressure-in the case of a low pressure gas, and in some instances; the feed gas for a single installationmay be from both high and low pressure-sources; vThecondensate is withdrawn from vessel 5; through conduit 6- andv this line may be controlled by the float control valve T. This condensa-te is conducted by conduit E to the fractionator- 3 where it will provide the reflux liquid for the fractionation step in a manner to be hereinafter more fullyl described.

The uncondensed portion of the Yfeed gas stream/isiconductedfrom vesselA 5 to absorber i through conduit and-enters below the trays of theabsorber. The absorption step may be of the conventional type with the lean oil introduced intothe upper portion-ofthe absorber through conduit-'9 and may be constituted of any desirable; absorbent menstruum. The lean oil and rising gases' pass in countercurrent now within the absorber and-the selectedheavier hydrocarbonconstituentsare dissolved fromvthe uprising gas-by the-downcoming absorbent medium. The undissolved vgases pass from the absorber through conduit Ill and maybe disposed of in any conventional manner; The rich absorbent menstruum; containing the heavy hydrocarbons from the feedstream'is'withdrawn from the bottom of the absorber through conduit ll and is conducted to amidpoint of rectier2. Heat is supplied to the lower portion of the rectifier as by a steam connection l2.

Intherectifier, the rich oil isseparated into the dissolved hydrocarbons which pass through conduit I3" as an overhead product and the rectied absorbent menstruum or lean oil, the latter being withdrawn from the lower portion of the rectiiier through'lin'e 9. This lean oil is circulated'by pump 1E; together with any lean oil from a reservoir which enters line 9` through conduit I5 "and is recirculated through the absorber. The stillfis shoinV scherhatically'withrthe reflux cir- 4 culating apparatus omitted in the interest of simplicity.

The overhead still product passes through line I3, cooler IS and condenser l1 into the fractionator 3. in the cooler and condenser, the stream is partially condensed and water is separated therefrom through trap lia and may be withdrawn throughline I'B controlled by a` suitable float valve.

l The partially condensed overhead still product enters the fractionator and the liquid portion flows' downwardly therein while the gaseous portion rises upwardlyin contact with the liquid reiiuiin'the'forn'of the condensate from vessel 5. In this gasandliquid contact, the selected heavy constituents of the upcoming gas are dissolved by thedown-c'omingliquid and replace the light constituents therefrom. The undissolved light constitutents pass as an overhead product through conduit I9 andare circulatedbyvcompresser 2i) tothe lower portion of the absorber in order that the undissolved selected heavy hydrocarbons escaping' therewithmayl be recovered.

In order to completely denude the overhead still product andthe condensate of the light'hy'fdroca'rbon material, it is necessary to introduce heat into the fractionator at the lowerportiriin the forni of a reboiler. It has beenfound that relatively low pressures may 'be employed within the fractionator and pressures rangin'gb'etween p. s. i. and 1 50 p; s; i. arerecomne'nde'd for this purpose. With such low pressures, a; 1oyv level heat maybe employed and desirably a'steam coil Z3 employing exhaust steam from the'plant is utilized for this purpose. It is"l usually desirable to employ a tower inter-cooler tool-'set the rise in temperature of the'downcoming liquid` due to the absorbed'he'at of vaporizatidn. Any suitable inter-coole'r may be employed for this prpose but it is preferredto utilize av liquid trap shown atta within the fractionator. This liquid trap' permits the' escape of 'rising gases thereby but collectsthedowncoiningliduid which is then circulated Vthroui'gh cooler 2'I'b`5T pu'iiip 22 andinirouiioooiiiio 'ino' tower iosii below-treo et; B'y this c'onibiriatnof tliefapIJ'litatnV r0f thh't to the bottom produce within uio frouoiiotor and the cooling of the dow'i'ficoiiningJ liquid within the fractionator, a proper'balarie may be' niyainioiiieuofiihin uio frootionatorso -osito completely denude thetw'o` hydrocarbon lstreamsenteringthe fractionator of uidfes''ir-able light constitetsto the extent' that addi ialstep's'alrd apparatus for their removal arelno rediiiid'. ,Y

This reuX condensate is readily av'ailabl'fin rather large quantitisin rst plaits and its constituency ondjiow temperature 'are' suon man it iondovitseif extremely Lwon `for use' as a 'rix iiquio from o.' foreign source iii inefrootitor. In accordance 'with thisfi'rvention, this c'ridensate is employed'to r'e'd'e the costv df initial equipment and also the c ost of operating-the froouonaior forwfieeiiigjuie selected nootyihydrocarbonsf 4f r'omth lghter'- material adding" in this fashion materially tothe ofiioiooy o'ftiie operation. ,1o addition it' iios heretofore' 'bo'e'n necessary-' to irtlif'e'r` treatJ this condensate" to remove the lighter material theiferoii and this is accomplished simiiuo'rieo'siy with uio' demethanization or deethani'zatin'of the s'till"ove'if headv product, thusfuadding' materially to the emciencyA oi thepr'ocessi Tpo lbottomfororiiici; ofjinoingoeiionator; which is-free'ofiiho iessvaiabie'ligiiier'motorino;are oiuiciiawifi the? 'i'fi'aftiotor`uiriugliy li'iio 24 and may either be utilized in this form or fur-4 ther separated into its various constituents in the manner well known to those skilled in thel be of any conventional character and may in-J clude a plurality of these towers. The residue gas passes from the absorber system through conduit I and the rich vabsorbent menstruum is conducted through pipe |I.to a suitable rectif-ier still where it is caused to give up its dissolved hydrocarbon constituents due 'to the action of the stripping steam from connection l 2. The lean oil from the bottom of the rectifier may be recirculated in the absorption system through conduit 9 and a proper quantity of oil may be kept in the system by introducing fresh oil as needed through conduit l from a suitable lean oil reservoir not shown in the drawings.

The hydrocarbons that have been dissolved by the absorbent menstruum in the absorber are released in the rectifier and pass as an overhead product through conduit I3, cooler I6 and condenser l'l into the fractionator., These hydrocarbon constituents are predominantly the desired heavy constituents but do contain an apprecable quantity of the lighter material. This material is advantageously and economically separated from the heavier constituents in fractionator 3 and at the same time, the condensate from vessel 5 is reconstituted or denuded of its lighter constituents within acceptable commercial limits. I

As heretofore indicated, the condensate from vessel 5 may, if desired, be flashed through successive pressure reduction stages to approximately the pressure prevailing within the fractionator 3. The vapors given olf during such flashing operations may be introduced into the absorption system in order to recover the heavy constituents thereof. This condensate is usually available in rather substantial quantities and must be subjected to some treatment to denude it of its lighter constituents. The condensate is obtained in a cold state and is advantageously employed as the reflux liquid being introduced into the fractionator through -conduit 6. In passing downwardly -through the column, the reflux liquid is brought into intimate gas and liquid contact with the upgoing vapors in the fractionator. 'I'hese vapors originate from the uncondensed portion of the rectifier overhead product and also from the release of vapors by the heater 23 which is effective to boil out all but acceptable traces of the lighter hydrocarbon material to be eliminated.

In order to elfect a proper balance Within the fractionator, it is usually necessary to cool the downgoing liquid in what is generally referred to as a tower inter-cooler. The proper balance between the quantity of reux liquid, the cooling in the inter-cooler, and the amount of reboiling due to heater 23 is eective to free the bottom productorthe fractionator tower of an but acceptable traces of its lighter constituents.

The overhead product from fractionator 3 will be composed primarily and predominantly of the lighter materials removed from the rectifier over-V head product and the condensate. However, this Y product, leaving the fractionator through linev I9, will contain some heavier hydrocarbons and in order to recover these, it is often desirable toV introduce this stream of gas into absorber A l near its bottom where it will pass upwardlyin countercurrentV relation with the downcoming absorbent menstruum.

In the fractionation of the rectier overhead product in the conventional manner, it is necessary to operate the fractionator at rather ele-V vat'ed pressures such as, for example, in the neighborhood of 550 p. s. i. 'I'he overhead prod uct from the fractionator is cooled and partially condensed to provide a reflux liquid which is then circulated back into the fractionator near its top. In accordance with this invention, the rectifier still overhead product may be fractionated at much lower pressures, as, for example, p. s. i. is preferred, and in the same operation, the

retrograde condensation product from the feed stream of gas is de-methanized or de-ethanized, as the case may be. This low pressure operation avoids the necessity of operating the fractionator reboiler in the region of the critical temperature of the product.

From the foregoing it will be seen that this invention is one well adapted to attain al1 of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the method.V Y

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed is:

1. A process for fractionating a natural gas feed containing methane and hydrocarbons heavier than methane to recover said heavier hydrocarbons substantially free, within acceptable limits, from said methane which comprises the steps of condensing a portion of said feed to obtain a condensate comprising a portion of said heavier hydrocarbons admixed with methane in an undesirably high concentration, contacting the remainder of said feed with absorption oil in an absorption zone to produce an absorption oil enriched with heavier hydrocarbons and methane from said remainder of said feed, passing said enriched oil to a rectifier and stripping it therein to produce a rectifier bottoms product comprising lean absorption oil and a rectifier overhead product comprising methane and at least a portion of said heavier hydrocarbons contained in said enriched oil, said methane being present in an undesirably high concentration in said rectifier overhead product, returning said lean absorption oil to said absorption zone, feeding said rectifier overhead product to a fractional distillation column, adding heat to said column at a point below the introduction of said rectiiier overhead product thereinto, reuxing said col- 

1. A PROCESS FOR FRACTIONATING A NATURAL GAS FEED CONTAINING METHANE AND HYDROCARBONS HEAVIER THAN METHANE TO RECOVER SAID HEAVIER HYDROCARBONS SUBSTANTIALLY FREE, WITHIN ACCEPTABLE LIMITS, FROM SAID METHANE WHICH COMPRISES THE STEPS OF CONDENSING A PORTION OF SAID FEED TO OBTAIN A CONDENSATE COMPRISING A PORTION OF SAID HEAVEIR HYDROCARBONS ADMIXED WITH METHANE IN AN UNDESIRABLY HIGH CONCENTRATION, CONTACTING THE REMAINDER OF SAID FEED WITH ABSORPTION OIL IN AN ABSORPTION ZONE TO PRODUCE AN ABSORPTION OIL ENRICHED WITH HEAVIER HYDROCARBONS AND METHANE FROM SAID REMAINDER OF SAID FEED, PASSING SAID ENRICHED OIL TO A RECTIFIER AND STRIPPING IT THEREIN TO PRODUCE A RECTIFIER BOTTOMS PRODUCT COMPRISING LEAN ABOSORPTION OIL AND A RECTIFIER OVERHEAD PRODUCT COMPRISING METHANE AND AT LEAST A PORTION OF SAID HEAVIER HYDROCARBONS CONTAINED IN SAID ENRICHED OIL, SAID METHANE BEING PRESENT IN AN UNDESIRABLY HIGH CONCENTRATION IN SAID RECTIFIER OVERHEAD PRODUCT, RETURNING SAID LEAN ABSORPTION OIL TO SAID ABSORPTION ZONE, FEEDING SAID RECTIFIER OVERHEAD PRODUCT TO A FRACTIONAL DISTILLATION COLUMN, ADDING HEAT TO SAID COLUMN AT A POINT BELOW THE INTRODUCTION OF SAID RECTIFIER OVERHEAD PRODUCT THEREINTO, REFLUXING SAID COLUMN SOLELY WITH SAID CONDENSATE INTRODUCED THEREINTO AT A POINT ABOVE WHICH SAID RECTIFIER OVERHEAD PRODUCT IS INTRODUCED THEREINTO, REMOVING AS AN OVERHEAD PRODUCT FROM SAID COLUMN A MIXTURE COMPOSED PREDOMINANTLY OF METHANE FROM SAID CONDENSATE AND METHANE FROM SAID RECTIFIER OVERHEAD PRODUCT, AND RECOVERING AS A BOTTOM PRODUCT FROM SAID COLUMN A MIXTURE OF SAID HEAVIER HYDROCARBONS FROM SAID CONDENSATE AND FROM SAID RECTIFIER OVERHEAD PRODUCT FREE, WITHIN ACEPTABLE LIMITS, FROM SAID UNDESIRABLE METHANE, SAID COLUMN HAVING SUFFICIENT OF SAID HEAT ADDED THERETO TO STRIP SAID BOTTOM PRODUCT OF THE UNDESIRABLE METHANE. 